Increase of the error rate in a data area attributable to a leakage magnetic field created by the residual magnetism of a magnetic disk is a problem in magnetic recording. In the perpendicular magnetic recording system, in particular, the influence of residual magnetism on the magnetic head is significant and hence the foregoing problem is important. The problem attributable to residual magnetism has become more important with the increasing demand for the perpendicular magnetic recording system.
For example, the residual magnetism of magnetic disks of the in-plane magnetic recording system and the perpendicular magnetic recording system is removed by an AC degaussing method. When the AC degaussing method is applied to degaussing, for example, a magnetic disk of the in-plane magnetic recording system, a magnetic field is applied in an in-plane direction parallel to the axis of easy magnetization of the recording layer of the magnetic disk with a permanent magnet or an electromagnet to invert the direction of the magnetic field so that the magnetism may be gradually reduced.
The residual magnetism of a magnetic disk of the perpendicular magnetic recording system is removed by applying magnetic fields of opposite polarities in a direction perpendicular to the surface of the magnetic disk, i.e., a direction parallel to the axis of easy magnetization of the recording layer of the magnetic disk, to invert the magnetism of the recording layer repeatedly so that the magnetism of the recording layer may gradually approach zero.
A data erasing method disclosed in Japanese Patent Publication No. 201-331904 (“Patent document 1”) combines magnets such that magnetic poles of opposite polarities are adjacent to each other and erases data recorded on the magnetic disk by leakage flux from the interface between the magnetic poles of opposite polarities. A data erasing device disclosed in Japanese Patent Publication No. 2005-276319 (“Patent document 2”) applicable to magnetic disks respectively of both the in-plane and the perpendicular magnetic recording system, uses a magnetic field varying from perpendicular direction at the inner circumference of the disk to a parallel direction at the outer circumference of the disk. A magnetic field applying method disclosed in Japanese Patent Publication No. 2004-326960 (“Patent document 3”) places magnets opposite to the upper and the lower surface of a magnetic disk, respectively, and slides the magnets along the recording surface of the magnetic disk.
A magnetic field of a magnetic intensity not lower than the coercive force of the magnetic disk is needed to degauss the magnetic disk. However, the recent magnetic recording systems including the perpendicular magnetic recording systems records data in a high recording density on magnetic disks and hence the coercive force of magnetic disks has been increased. Accordingly, a magnetic field of a high magnetic intensity needs to be applied to the magnetic disk for high-density recording. For example, it is difficult to degauss a magnetic disk of the perpendicular magnetic recording system by applying a magnetic field of the same magnetic intensity as that of a magnetic field used by a degaussing device for degaussing a magnetic disk of the in-plane magnetic recording system by a degaussing device similar to the degaussing device for the in-plane magnetic recording system.